Avoid write-after-write issue hazards for Cortex-A9.

Add a avoidWriteAfterWrite() target hook to identify register classes that
suffer from write-after-write hazards. For those register classes, try to avoid
writing the same register in two consecutive instructions.

This is currently disabled by default.  We should not spill to avoid hazards!
The command line flag -avoid-waw-hazard can be used to enable waw avoidance.

llvm-svn: 129772

1 files changed, 23 insertions, 0 deletions

diff --git a/llvm/lib/Target/ARM/ARMBaseRegisterInfo.cpp b/llvm/lib/Target/ARM/ARMBaseRegisterInfo.cpp
index 3d1eaf0891a..6eb9002df83 100644
--- a/llvm/lib/Target/ARM/ARMBaseRegisterInfo.cpp
+++ b/llvm/lib/Target/ARM/ARMBaseRegisterInfo.cpp
@@ -554,6 +554,29 @@ ARMBaseRegisterInfo::UpdateRegAllocHint(unsigned Reg, unsigned NewReg,
   }
 }
 
+bool
+ARMBaseRegisterInfo::avoidWriteAfterWrite(const TargetRegisterClass *RC) const {
+  // CortexA9 has a Write-after-write hazard for NEON registers.
+  if (!STI.isCortexA9())
+    return false;
+
+  switch (RC->getID()) {
+  case ARM::DPRRegClassID:
+  case ARM::DPR_8RegClassID:
+  case ARM::DPR_VFP2RegClassID:
+  case ARM::QPRRegClassID:
+  case ARM::QPR_8RegClassID:
+  case ARM::QPR_VFP2RegClassID:
+  case ARM::SPRRegClassID:
+  case ARM::SPR_8RegClassID:
+    // Avoid reusing S, D, and Q registers.
+    // Don't increase register pressure for QQ and QQQQ.
+    return true;
+  default:
+    return false;
+  }
+}
+
 bool ARMBaseRegisterInfo::hasBasePointer(const MachineFunction &MF) const {
   const MachineFrameInfo *MFI = MF.getFrameInfo();
   const ARMFunctionInfo *AFI = MF.getInfo<ARMFunctionInfo>();